Because each Outlook version uses its own version of MAPI, you cannot install more than one version of Outlook on any machine. This is also true for anything else that requires MAPI components like BES or Exchange 2003. If you try to install Outlook on a machine with BES or Exchange 2003, you break something because of the MAPI version mismatch.

Bad news my friend... Microsoft in the Article says it cannot be done with outlook 2010. Therefore you will need to setup a VM to test with.

Excerpt from the article:

Multiple versions of Outlook

Microsoft
Office Outlook 2010 cannot coexist
with any earlier version of Microsoft
Outlook. When you install Outlook
2010, the Setup program removes
Microsoft Office Outlook 2007,
Microsoft Office Outlook 2003, and
Microsoft Outlook 2002. The Setup
program removes these versions of
Outlook even if you click to select
the Keep these programs check box in
the Removing Previous Versions dialog
box.

Trying to have things coexist, that aren't really 'supposed' to, for testing purposes, is at best inconvenient (because of all the messing around you often have to do), and at worst misleading (because it's sometimes not obvious that the messing you've had to do to achieve the coexistence, also changes the behaviour in some subtle way, that invalidates the test).

Been there, bought the t-shirt, experienced the pain.

Much better to virtualise - then you can have as many test environments as you like, all different, and each uncontaminated by the others.